When testing closed containers one known technique is to arrange a container to be tested into a test cavity which is then sealingly closed, then to evacuate the interior space of the test cavity around the container to be tested and to evaluate the time behaviour of the pressure in the surrounding of the container after evacuation has been stopped at a predetermined level. Although this technique is of very high accuracy it necessitates utmost care for reaching such high accuracy. The volume of the test cavity and its shape must snugly fit the outside shape of the container to be tested. On one hand minimising this volume leads to respectively short evacuation time, on the other hand the degree of this minimising largely governs the detection accuracy reached. As a change in pressure in the surrounding of the container is detected as leak indication entity, the smaller than the volume is in which, through a leak, pressure is affected, the higher will be the detection accuracy.
Further, accuracy is largely influenced by the degree of vacuum which is established in the surrounding of the container, which makes it necessary, for high accuracy, to provide relatively expensive vacuum pumps, possibly even multiple stage vacuum pumps, if vacuum is to be established down to the level as only reached with turbo vacuum pumps.